Proton Synchrotron

The Proton Synchrotron (PS) is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It has since served as a pre-accelerator for the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS), and is currently part of the Large Hadron Collider (LHC) accelerator complex. In addition to protons, PS has accelerated alpha particles, oxygen and sulphur nuclei, electrons, positrons and antiprotons. The Proton Synchrotron (PS) is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It has since served as a pre-accelerator for the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS), and is currently part of the Large Hadron Collider (LHC) accelerator complex. In addition to protons, PS has accelerated alpha particles, oxygen and sulphur nuclei, electrons, positrons and antiprotons. Today, the PS is part of the CERN's accelerator complex. It accelerates protons for the LHC as well as a number of other experimental facilities at CERN. Using a proton source, the protons are first accelerated to the energy of 50 MeV in the linear accelerator Linac 2. The beam is then injected into the Proton Synchrotron Booster (PSB), which accelerates the protons to 1.4 GeV, followed by the PS, which pushes the beam to 25 GeV. The protons are then sent to the Super Proton Synchrotron, and accelerated to 450 GeV before they are injected into the LHC. The PS also accelerate heavy ions from the Low Energy Ion Ring (LEIR) at an energy of 72 MeV, for collisions in the LHC. The synchrotron (as in Proton Synchrotron) is a type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed path. The magnetic field which bends the particle beam into its fixed path increases with time, and is synchronized to the increasing energy of the particles. As the particles travels around the fixed circular path they will oscillate around their equilibrium orbit, a phenomenon called betatron oscillations. In a conventional synchrotron the focusing of the circulating particles is achieved by weak focusing: the magnetic field that guides the particles around the fixed radius decreases slightly with radius, causing the orbits of the particles with slightly different positions to approximate each other. The amount of focusing in this way is not very great, and consequently the amplitudes of the betatron oscillations are large. Weak focusing requires a large vacuum chamber, and consequently big magnets. Most of the cost of a conventional synchrotron is the magnets. The PS was the first accelerator at CERN that made use of the alternating-gradient principle, also called strong focusing: quadrupole magnets are used to alternately focus horizontally and vertically many times around the circumference of the accelerator. The focusing of the particle can in theory become as strong as one wishes, and the amplitude of the betatron oscillations as small as desired. The net result is that you can reduce the cost of the magnets. When early in the 1950s the plans for a European laboratory of particle physics began to take shape, two different accelerator projects emerged. One machine was to be of standard type, easy and relatively fast and cheap to build: the Synchrocyclotron, achieving collisions at a center-of-mass energy of 600 MeV. The second device was a much more ambitious undertaking: an accelerator bigger than any other then existing, a synchrotron that could accelerate protons up to an energy of 10 GeV — the PS. By May 1952 a design group was set up with Odd Dahl in charge. Other members of the group were among others Rolf Widerøe, Frank Kenneth Goward and John Adams. After a visit to the Cosmotron at Brookhaven National Laboratory in the US, the group learnt of a new idea for making cheaper and higher energy machines: alternating-gradient focusing. The idea was so attractive that the study of a 10 GeV synchrotron was dropped, and a study of a machine implementing the new idea initiated. Using this principle a 30 GeV accelerator could be built for the same cost as a 10 GeV accelerator using weak focusing. However, the stronger focusing the higher a precision of alignment of magnets required. This proved a serious problem in the construction of the accelerator. A second problem in the construction period was the machines behavior at an energy called 'transition energy'. At this point the relative increase in particle velocity changes from being greater to being smaller, causing the amplitude of the betatron oscillation to go to zero and loss of stability in the beam. This was solved by a jump, or a sudden shift in the acceleration, in which pulsed quadruples made the protons transverse the transition energy level much faster. The PS was approved in October 1953, as a synchrotron of 25 GeV energy with a radius of 72 meter, and a budget of 120 million Swiss franc. The focusing strength chosen required a vacuum chamber of 12 cm width and 8 cm height, with magnets of about 4000 tonnes total mass. Dahl resigned as head of the project in October 1954 and was replaced by John Adams. By August 1959 the PS was ready for its first beam, and on 24 of November the machine reached a beam energy of 24 GeV. By the end of 1965 the PS was the center of a spider's web of beam lines: It supplied protons to the South Hall (Meyrin site) where an internal target produced five secondary beams, serving a neutrino experiment and a muon storage ring; the North Hall (Meyrin site) where two bubble chambers (80 cm hydrogen Saclay, heavy liquid CERN) were fed by an internal taget; when the East Hall (Meyrin site) became available in 1963, protons from the PS hit an internal target producing a secondary beam filtered by electrostatic separators to the CERN 2 m bubble chamber and additional experiments.